Thin film photovoltaic devices may contain several material layers deposited sequentially over a substrate, including semiconductor material layers which form a p-type absorber layer and an n-type window layer. Vapor deposition is one technique which can be used for depositing semiconductor material layers over a substrate. In vapor deposition a semiconductor material in solid form is vaporized under high temperatures with the vapor flow being directed towards a substrate where it condenses on the substrate as a thin solid film. One such vapor deposition technique is known as vapor transport deposition (VTD). An example of a known vapor transport deposition system can be found in U.S. Pat. No. 5,945,163. In a VTD system, as shown in U.S. Pat. No. 5,945,163, a semiconductor material in a powder form, is continuously supplied to the interior of a permeable vaporization chamber with the assistance of a carrier gas. The vaporization chamber is heated to a high temperature sufficient to vaporize the powder, with the vapor passing through a permeable wall of the vaporization chamber. The vapor is then directed by a distributor towards, and condenses as a thin film on, a substrate which moves past one or more orifices of the distributor which direct the vapor towards the substrate.
In order to achieve a high production line throughput, each semiconductor material is deposited in a single stage deposition as a single layer on the substrate to a desired thickness. To achieve the desired thickness with a high production speed, a large volume of semiconductor powder must be vaporized in a short time which requires that the semiconductor powder be heated to a high temperature in the vaporization chamber. Temperatures typically used for VTD deposition are in the range of about 500° C. to about 1200° C., with higher temperatures in this range being preferred for a high deposition throughput. The vaporization chamber can be formed as a heatable tubular permeable member formed of silicon carbide (SiC). The distributor can be formed of a shroud of ceramic material, such as mullite. Vapor deposition occurs within a housing which contains a substrate transport mechanism such as driven rollers. Ceramic sheets may also be used as heat shields within the housing. When the semiconductor material to be deposited contains tellurium, vaporization at the higher temperature can cause materials of the tubular permeable member, the mullite shroud, ceramic sheets and other equipment associated with the deposition to also vaporize and chemically react with tellurium to form a tellurium chemical species vapor which can be deposited with the tellurium containing semiconductor material. This, in turn, leads to undesired impurities being present in the deposited semiconductor film as a contaminant. Some of these impurities may include one or more of tantalum (Ta), cobalt (Co), copper (Cu), vanadium (Va), iron (Fe), antimony (Sb), zirconium (Zr), tin (Sn), silicon (Si) and aluminum (Al). If the impurities have a high enough concentration in the deposited film, they may adversely affect the electrical performance of the tellurium containing semiconductor material.
On the other hand, it may be desirable to add dopants during the vapor deposition of the tellurium containing semiconductor material to achieve a desired dopant concentration in the deposited films. As an example, silicon (Si) has been used as a dopant in some tellurium containing semiconductor films. However, it is difficult to control the silicon (Si) dopant concentration in the deposited film if silicon (Si) is also uncontrollably being introduced as an impurity vapor from the heated silicon carbide (SiC) tubular permeable member during high temperature deposition. For an impurity such as silicon (Si) which may be desired in the deposited tellurium containing semiconductor material as a dopant the silicon atomic concentration should be limited to the range of about 1 e16/cm3 to about 1 e18/cm3 in a deposited film. This will ensure that silicon is present at a level to be effective, but not at a level which may be detrimental to operation of the deposited tellurium containing semiconductor material. In addition, any individual impurity, whether desired or not, should be limited to have an atomic concentration of less than or equal to about 1 e18/cm3 and also be uniformly distributed in the deposited tellurium containing semiconductor material. Vapor deposition at higher temperatures can result in a non-uniform impurity distribution in the deposited film.
Accordingly, a method and apparatus for better controlling the amount of impurities which are being vaporized, reacted with tellurium and incorporated into a deposited tellurium containing semiconductor thin film is desired.